The present invention relates to guidance apparatus and, more particularly, to guidance apparatus having a dual electro-optical and radio frequency sensing capability especially suitable for tactical guided missiles.
Tactical guided missiles are used in a variety of military applications to provide the capability to attack targets in the air and on the surface. In each application, some type of target sensing capability is required. Known sensing apparatus includes both active systems which generate and transmit various types of radiation and develop guidance signals based on reflections obtained from a target, and passive systems which develop guidance signals from radiation generated directly by the target itself.
The type of radiation sensed in both active and passive systems is dependent upon the particular application. For example, some systems known in the art have the capability to sense radiation in the visible electro-optical spectrum (i.e., television systems). Other known systems sense infrared radiation generated by the exhaust from the engines of aircraft or armored vehicles, or radio frequency radiation of various wavelengths either generated or reflected by the target.
Each type of sensor and each radiation frequency has advantages and disadvantages. For example, extremely short wavelength radio frequency active systems (commonly known as millimeter-wavelength radar systems) are able to provide guidance to targets in all weather, to accurately determine the distance to a target, and exhibit insensitivity to infrared countermeasures. However, such active radio frequency systems provide limited discrimination between multiple targets, are limited in angular resolution, and are unable to provide guidance information at very close distances to the target. On the other hand, passive electro-optical guidance systems provide good target discrimination and high resolution of target details, and are able to provide guidance signals almost up to the point of impact with the target. Such passive electro-optical systems are immune to radio frequency jamming but are ineffective under conditions of poor visibility from, for example, clouds, precipitation, or dust. Furthermore, they are unable to provide information regarding the distance to the target, and are susceptible to countermeasures such as exhaust shielding and decoy flares.
It can be seen that the characteristics of active radio frequency guidance systems and passive electro-optical guidance systems can be complementary. Accordingly, a first objective of the invention is to provide guidance apparatus employing both an active radio frequency guidance system and a passive electro-optical guidance system.
Various systems for providing such a dual-mode guidance apparatus are known in the prior art. One approach has been to combine two existing guidance systems, such as placing the infrared guidance sensor of the Redeye missile inside the active radio frequency seeker of the Hawk missile. Another similar effort involved the placing of an "eggcrate" radio frequency antenna behind a separate infrared sensor. Yet another dual-mode system in the prior art employs a forward looking infrared seeker under one protective cover (or aperture) in conjunction with a side-looking radio frequency system under another aperture. Although some benefits are obtainable using such a segmented aperture system, the two sensors do not view the target in the same perspective, and pointing problems may occur in the changeover from one system to another, since the sensors are not looking at the same plane.
Another approach to a dual-mode sensor has been proposed using radio frequency optics having dichroic elements. Dichroic elements selectively transmit or reflect radiation, depending upon the frequency of the radiation. Suitable dichroic elements are known which can perform over a frequency separation of several octaves; that is, one frequency is a multiple of two or three times the other frequency. However, dichroic elements are not yet available which can effectively operate over the wavelength ratios of well over one thousand that a dual-mode millimeter wave radio frequency/electro-optical guidance system would require.
Microstrip radio frequency antennas provide attractive features for use in a dual-mode guidance system for a tactical guided missile. Such antennas employ a large number of flat antenna elements arranged in an array. However, conventional microstrip antennas on polymeric microwave substrates are difficult to form into the type of optical configurations which are desirable for use with an infrared sensor. Furthermore, conventional printed circuit board fabrication techniques used in microstrip antennas have limitations in the gain which can be achieved, due to surface roughness of the elements.
Accordingly, another objective of the invention is to provide guidance apparatus which will provide a dual-mode sensing capability including an active radio frequency guidance system and a passive electro-optical guidance system which can utilize a common aperture and provide coincident detection beams for both the radio frequency system and the electro-optical system. Yet another objective of the invention is to provide such a system which is mechanically rugged yet which exhibits high gain and efficient performance.